Laser desorption coupled to laser multiphoton-ionisation mass spectrometry is used to study spontaneous or laser induced NH3 reactivity on silicon surface at room temperature. A 355 nm laser, with a 4.1 ns pulse duration, is focused on a silicon surface pre-exposed to ammonia with laser energy under ablation threshold. Depending on laser irradiation energy there is desorption of species or bond modification leading to desorption of more molecules. With soft laser energy, we only detect desorbed neutral SiNH2 molecules. This molecule is the signature of a dissociative adsorption of NH3 on Si. From a critical laser energy E-m laser irradiation induces surface melting and we detect other desorbed species Si-2, Si-NH2 and Si-2-NH which are characteristic of new bonding on surface. E-m represents the energy threshold for silicon nitriding induced by laser. (C) 2002 Elsevier Science B.V. All rights reserved.